In casting hot-formed copper-base products it is desirable to control the grain structure within the cast bar. Some applications call for a product having fine equi-axed grains with no grain alignment, while others may be suitably produced having a defined dendritic structure.
These grain structures may be obtained during the solidification of molten metal while closely controlling the solidification parameters.
In the casting operation known as wheel and belt casting, molten metal is fed into a groove cut into the periphery of a circular mold, a flat belt being pressed around a portion of the periphery of said mold forms the solidification chamber. Molten metal is fed into the mold at the point where the flat belt first contacts the wheel, with solidification taking place as heat is removed from the molten metal by the mold.
The rate at which heat is removed from the molten metal is controlled by a number of variables. One of these variables is the location of cooling water or cooling spray applied to the mold itself. Another variable is the quantity of cooling liquid applied to the mold, and still another variable being the placement and rate of application of cooling liquid applied to the belt forming the cavity.
A cross section of the casting ring or casting mold reveals it to be basically a U-shaped mold with approximately equal mass on three sides. This mold is typically made of a copper or copper based alloy and is an excellent heat conductor. The belt used to enclose the U-shaped mold and create the closed side of the casting cavity is typically a thin steel band. Steel is a much poorer conductor of heat than copper. For these reasons, it may readily be seen that molten metal contained on three sides by a thick, high heat transferring mold medium and on the fourth side by a thin, poor heat transferring medium, a non-uniform rate of heat removal would be expected from the four sides of the molten metal. In an alloy or nearly pure copper product in which an equi-axed grain structure is obtained, this non-equal removal of heat from the molten metal is not critical. However, in a cast bar having a columnar or dendritic grain structure, it is most desirable that heat is removed from the metal at approximately equal rates from opposite sides of the bar. That is, the removal of heat from the left and right side of the cast bar should be approximately equal, and the heat removed from the upper and lower side of the bar should be equal. This will cause the dendrites to grow at an equal rate, thereby completing the solidification process at approximate the center of the cast bar. If the bar were perfectly square, the dendritic pattern would appear to grow equally from all four sides to a point in the center of the bar. In the situation where the bar is wider than it is tall, the dendritic solidification pattern will appear to be a small triangular shaped dendritic pattern pointing toward the center of the bar from each of the two short sides and a trapezoidal shaped dendritic pattern growing from each of the two long sides resulting in a long straight interface between the dendritic grains growing from the two longer solidification fronts.
As noted before, the equal removal of the heat, absent some means of modifying the natural heat removal in the above described system, is not readily obtained. Heat will be removed from the three sides bound by the heavy copper mold at a faster rate than will be removed through the thin metal band. This will result in the solidification interface being shifted much to the band side of the copper cast bar with a very short line of dendrites on the band side and extremely long dendrites having grown from the copper wheel side. This results in a copper bar whose dendritic pattern is not symmetrical and whose working characteristics will not be desirable.